Bodies were found strewn around the maximum-security wing, the kitchen, a prison yard and a conjugal visits area after the pre-dawn riot at the Las Cruces federal prison, said the security spokesman for the state of Guerrero, Roberto Alvarez.
"The incident was triggered by a permanent feud between rival groups within the prison," he told a press conference.
State police have regained control of the prison, backed by federal police and the army, which set up a security cordon outside, he said.
Officials initially gave a death toll of five. Alvarez had earlier said that victims were stabbed and beaten to death.
More From This Section
Guards said some of the victims had their throats slit, according to an internal state police report. It also said gunshots had been fired in the maximum-security wing, apparently by prisoners.
There was a heavy security presence outside the prison, with soldiers and riot police encircling the compound and two helicopters hovering overhead.
Dozens of anxious relatives had gathered, demanding news on their loved ones.
Mexico's chronically overcrowded prisons are frequently hit by riots and jailbreaks.
Inmates have de facto control over many of Mexico's jails.
Contraband weapons and drugs are rife, and experts say organized crime networks often operate from inside prison walls.